1) Technical Field of the Invention
This invention relates to a light emitting device made of nitride compound semiconductor of III-V group (in general, expressed in a formula of InxAlyGa1-x-yN, wherein 0 less than X, 0xe2x89xa6Y, X+Yxe2x89xa61). And especially it relates to a light emitting device including an active layer depositing at least two kind of well layers emitting different colors of light and mixing colors thereby emitting light of another color such as white with a desired color rendering property.
2) Description of Related Art
Since light emitting devices which emit light of red, green, and blue color (so-called RGB) with quite strong intensity have been developed so far, RGB light emitting devices of high luminous intensity type are now manufactured and available. It is noted that the color of light emitted by the nitride compound semiconductor device can be varied from ultraviolet region through red region, by adjusting the composition ratio thereof.
Meanwhile, because of excellent characteristics of the light emitting devices such as high luminous intensity, downsizing, and high credibility, applications thereof have been rapidly expanding in the technical fields, for example, for light sources for indicators mounted on automobiles, light sources for backlights of liquid crystal displays, and other luminaires.
In those applications, especially since white light emitting devices are easiest and most comfortable to human eyes, such white light emitting devices are highly desired. There are basically two approaches to obtain such white light. Three of RGB or two of blue and yellow (that is complementary color of blue) color light emitting diode chips are mounted on a same stem to mix them to produce the white light. Alternatively fluorescer emitting fluorescent yellow light when absorbing blue light are applied on the blue color emitting diode chips to produce white light.
However those approaches to achieve white light have some problems. When mounting a plurality of light emitting diode chips on the same stem to produce white light, those chips have to be disposed as closely as possible each other in order to improve a color mixing property. (The color mixing property is referred herein denotes the extent to which the light from the device can be evenly seen as single white light.) But the finite sizes of the chips limit the improvement. Further when utilizing chips of different semiconductor based materials, they have different forward voltages and distinct characteristics dependent upon the temperature.
Also when utilizing fluorescer to produce the white light, a step for applying fluorescer on the chip is required, which is a complicated step. Further when combining the blue light emitting chip and fluorescer emitting fluorescent yellow light by absorbing the blue light to obtain white light, the luminous efficiency is theoretically reduced, in comparison with the combination of a plurality of different color chips.
Therefore those conventional approaches are not yet satisfactory to replace the current light sources with the semiconductor light emitting devices, and it is desired to provide the light emitting devices with higher luminous efficiency and higher luminous intensity that are capable of emitting white light. This invention is directed to providing such light emitting device that can emit white light by itself.
The first object of the present invention is to provide a light emitting device including at least two well layers made of nitride compound semiconductor emitting different color light due to different In composition ratios to so that white light can be obtained by mixing the different color light.
The second object of the present invention is to provide a light emitting device including at least one of first and second well layers where the second well layer has rougher surface than that of the first well layer to improve a luminous efficiency.
The third object of the present invention is to provide a light emitting device further including a first barrier layer containing Al and second barrier layer substantially not containing Al, which are formed on each well layer to reduce the forward voltage thereby improve the luminous efficiency.
The light emitting device according to an aspect of the present invention, comprises an active layer of a multiple quantum well structure, sandwiched between an n-type semiconductor layer and a p-type semiconductor layer; the active layer including, at least one of first well layers made of nitride compound semiconductor containing In, and at least one of second well layers made of nitride compound semiconductor containing In, the second well layer emitting light having a principal peak wavelength longer than that of the first well layer.
When the principal peak wavelength of the first and second well layers are selected such that they are complementary colors each other, and thus a white color light can be obtained by mixing the two color lights.
The light emitting device according to another aspect of the present invention, is characterized in that the second well layer is disposed between the first well layer and the p-type semiconductor layer.
In general, it is very difficult to grow the nitride compound semiconductor containing In. As the In composition ratio within the well layer is higher, the light emitted by the well layer has a longer wavelength and its crystallinity is worse so that the luminous efficiency is also reduced. This property is remarkable when the In composition ratio is not less than 0.05. In fact, the diffusion length of the hole is very short within the nitride compound semiconductor of quantum well structure. Therefore it is, preferable to deposit the second well layer adjacent to the p-type semiconductor layer that provide hole because the second well layer has less the luminous efficiency due to the more In composition ratio and worse crystallinity. In other words, this is because the hole-electron recombination possibility near the p-type semiconductor layer is higher than that far from the p-type semiconductor layer. Thus the luminous efficiency of the device can be improved by depositing the second well layer between the first well layer and the p-type semiconductor layer.
The light emitting device according to another aspect of the present invention, is characterized in that growth numbers of the first and second well layers are adjusted to control the luminous intensity ratio of light emitted by the first well layer over light emitted by the second well layer. Thus a desired color rendering property can be obtained. (The color rendering property is referred herein as to one of the effects of a light source under which an object can be seen.)
So far, most of light sources rather than light emitting semiconductor devices have been also developed to realize many kinds of color rendering property: For instance there are many types of fluorescent lamps illuminating with white light of different nuance. Therefore the light emitting device according to the present invention is desired to easily adjust its color rendering property to replace the conventional light sources.
According to the present invention, its color rendering property, in other words, the ratio of luminous intensity emitted by each well layer is adjusted by controlling the ratio of the growth numbers of the first and second well layers. Rather than this approach, as shown in Japanese Laid-Open publication 10-22525, the ratio of luminous intensity emitted by each well layer can be adjusted by controlling thickness of each well layer. However it has a problem. When its active layer is made of quantum well structure, as the well layer is thinner, although the luminous intensity is stronger to some extent, the peak wavelength shifts to shorter side due to the quantum size effect. Meanwhile as the well layer is thicker, although the luminous intensity is weaker to some extent, the peak wavelength shifts to longer side. That is to say, it is difficult to achieve light having the just desired peak wavelength.
The light emitting device according to another aspect of the present invention, is characterized in that the device further comprising a plurality of barrier layers sandwiching said first and second well layers; wherein thickness of said first and second well layers are adjusted to control a luminous intensity ratio of light emitted by said first well layer over light emitted by said second well layer thereby obtaining the device having a desired color rendering property.
As mentioned above, its color rendering property (nuance of the light source), in other words, the ratio of luminous intensity emitted by each well layer can be adjusted by controlling not only the ratio of the growth numbers but also the thickness of the first and second well layers.
The light emitting device according to another aspect of the present invention, is characterized in that the growth number of the first well layer is more than that of the second well layer.
Especially when the second well layer is disposed between the first well layer and the p-type semiconductor layer, light emitted by the first well layer is absorbed in the second well layer so that its luminous intensity of the first well layer is reduced. Therefore the light emitting device with desired color rendering property can be easily obtained by depositing the first well layers more times than the second well layer.
The light emitting device according to another aspect of the present invention, is characterized in that a light spectrum emitted by the first well layer has a half-width narrower than that emitted by the second well layer.
When the device includes the first well layer emitting light having principal wavelength in the blue region and the second well layer emitting light having principal wavelength in the yellow region, the mixed light doesn""t often show a good color rendering property because it includes no or little light in the green and red region. Therefore it is preferable that the beam spectrum emitted by the second well layer is broader than that of the first well layer. Thus the general color rendering index Ra of light emitted by the device can be improved without reducing the luminous efficiency, that is, the light emitting device with high luminous intensity and high color rendering property can be obtained. It would be understood by the persons skilled in the art that the half-width of the light spectrum can be controlled, for instance, by adjusting the crystallinity of the well layers or adding impurity therein.
The light emitting device according to another aspect of the present invention, is characterized in that the growth number of the first well layer falls within the range of 2 through 10, and the growth number of the second well layer falls within the range of 1 through 3.
The light emitting device according to another aspect of the present invention, is characterized in that the principal peak wavelength of the first well layer falls within the range of 450 through 500 nm, and the principal peak wavelength of the second well layer falls within the range of 560 through 670 nm.
The light emitting device according to another aspect of the present invention, is characterized in that each of growth surfaces of said first well layers is rougher than that of each of said second well layer.
In general, it has been understood that where the light emitting device has the well layer of the multiple quantum well structure, which has smoother growth surfaces (flatter composition face with barrier layer) and better crystallinity, then its luminous efficiency is improved. However the inventor of the present invention found that it is not always true, especially in case where there may be a certain interactive effect between the first and second well layers adjoining over the barrier layer. Thus the present inventor has firstly recognized that in the case of that the degree of asperity of the second well layer emitting the longer wavelength light is greater than that of the first well layer emitting the shorter wavelength light, the shorter wavelength light from the first well layer is less absorbed in the second well layer. Moreover the inventor has found that under this condition, the degree of asperity of the second well layer can be optimized as explained below to improve the luminous efficiency of the second well layer also, thereby improving the luminous efficiency of the light emitting device in total.
Now an example how to compare the degree of the asperity of the first and second well layers is described hereinafter.
Cutting down the light emitting device in the direction perpendicular to the layer thickness, the degree of asperity of the composition faces between the well layers and barrier layer can be observed and compared in the cross section through a scanning electron microscope. The difference of degree of asperity or roughness of the first and second well layers is often distinguished by observing through the microscope. But in the case where such difference is not clear, the degrees of asperity may be compared by measuring the index R which will be described hereinafter. Alternatively in the manufacturing process of the device, the degree of asperity may be measured by using a GIXR (Grazing Incidence X-Ray Reflection) method for at least one of the upper and lower surfaces. Where there are plural of the first and/or second well layers, at least one of the second well layers has a greater degree of asperity than that of at least one of the first well layers. But preferably all of the second well layers have a greater degree of asperity than that of all of the first well layers. It is noted that even in the case where there are plural of the first and/or second well layers, since the crystallinity and the growth configuration of the well layers depends mainly upon the In composition ratio, each of the first and second well layers has the almost same degree of asperity.
Further forming the second well layer having greater degree of asperity than that of the first well layer is preferable in the view point of the crystallinity of the second well layer. That is to say, in the case where the second well layer has greater degree of asperity than that of the first well layer (including the case where only the second well layer has asperity), the crystallinity of the second well layer is much worse than that in the case where the first well layer has greater degree of asperity than that of the second well layer. In the later case, it is difficult to grow the second well layer which has even thickness in some particular portions across the wafer, and to be worse, the second well layer may not be grown in some region at all. On the contrary, where the second well layer has greater degree of asperity than that of the first well layer, the second well layer can be grown having comparatively even thickness. Therefore it is understood that the characteristics of the light emitting device is improved by making the degree of asperity of the second well layer greater than that of the first well layer. Such grown device, as indicated above, has the first and second well layers both having good crystallinity and even thickness. Consequently, the ratio of the luminous intensity emitted by the first and second well layers is kept constant, that leads an improvement in minimizing diversity of the luminous intensity and luminous nuance between each device.
As it is clearly understood by the persons skilled in the art that such degree of asperity of the first and second well layers can be adjusted by controlling the growth condition such as the growth speed and the growth temperature. The degree of asperity may be controlled by any other growth condition than the above-mentioned.
Where the second well layer has a greater degree of asperity than that of the first well layer, the luminous efficiency is improved. It is not clear for the exact relation how each degree of asperity of the first and second well layers influences the luminous efficiency. But it is understood that such influence to the luminous efficiency by the first well layer emitting shorter wavelength light is different from that by the second well layer emitting longer wavelength light. In other words, the device characteristics is influenced by the In composition ratio, the growth number, the growth order of the first and second well layers of the quantum well structure in conjunction with the degree of asperity of each well layer. Since the degrees of asperity of the first and second well layers give different influence to the luminous efficiency, the optimum degree of asperity of first well layer is different from that of the second well layer.
In addition, where the degree of asperity of the second well layer is greater than that of the first well layer, since the luminous intensity emitted by each well layer is kept constant, the light source having desired color rendering property is easily obtained. This comes principally from the good crystallinity as described above. Further where the second well layer has some degree of asperity, more light emitted by the first well layer can come through the second well layer, and the first well layer produces more light (stronger luminous intensity). The second well layer also shows the similar tendency even to less extent though. As explained above, the degree of asperity and the relation thereof cooperatively improve the total luminous efficiency of the device so that the light source having desired color rendering property is easily obtained.
The light emitting device according to another aspect of the present invention, is characterized in that each of the second well layers has dished portions having thickness less than a half of an average thickness thereof, and an area of the dished portions is not less than 10% thereof.
Now referring to FIG. 6, an example of steps for the measuring the ratio of the area of the dished portions over the total surface of the well layer 108, 109. A first step is cutting down the light emitting device in the direction perpendicular to the layer thickness, and observing the cross section of the well layer through a scanning electron microscope or the like. The well layer includes dished portions D having thickness less than a half of an average thickness thereof. The total length S of the dished portions D are counted over a certain region (length L). Then the degree R of the area S of the dished portions over the total surface L is calculated by the expression of R=S/L. Thus the degree R is defined as a ratio of the dished portions occupied in the total surface. Although it is sufficient that the total surface L to be measured is about 1 xcexcm, preferably it is more than about 10 xcexcm, then the degree R can be measured repeatedly correctly. The average thickness of the well layer across the total surface may be measured by measuring the average thickness over the certain region L of the well layer. Further since the degree R of the well layer across the total surface is, in general, to be constant within the device, the R can be measured at any cross section.
In other words, this aspect of the light emitting device according to the present invention is characterized in that the second well layers has the degree R not less than 0.1. Where the degree R of the second well layer is less than 0.1, that is to say, where the second well layer has a relatively flat surface, then light emitted by the first well layer is reduced in comparison with light emitted by the second well layer, and the total luminous efficiency of the light emitting device is also reduced.
Where a plurality of the second well layers are deposited, at least one of the second well layers satisfying the above condition Rxe2x89xa70.1 leads the improvement of the luminous efficiency of the first well layer, but preferably all of the second well layers satisfy the above condition.
Where the degree R of the second well layer is over than 0.5, the p-type semiconductor layer formed on the second well layer loses the flatness so that the luminous efficiency is reduced. Therefore if the degree R of the dished portions occupied in the surface of the second well layer has an upper limit of 0.5, light emitted by the first well layer efficiently comes through the device so that the luminous characteristics of the light emitting device is maintained satisfactory.
The light emitting device according to another aspect of the present invention, is characterized in that the second well layer having the degree of asperity greater than that of the first well layer, or having the degree R not less than 0.1 is disposed between the first well layer and the p-type semiconductor layer.
As described above, in case of that the second well layer having a higher In composition ratio and relatively worse crystallinity is disposed more adjacent to the p-type semiconductor layer (where the hole-electron recombination possibility is higher), advantageously the luminous efficiency can be improved.
Where the first well layer 108 has the degree of asperity greater than that of the second well layer, the light emitted by each device in a wafer shows a great diversity in wavelength. Such great diversity in wavelength among devices makes difficult to repeatedly and reliably manufacturing the light emitting devices having a desired color rendering property. Thus it is preferable also in view of this point to deposit the second well layer between the first well layer and the p-type semiconductor layer.
The light emitting device according to another aspect of the present invention, is characterized in that the growth number of said first well layer is more than that of said second well layer.
Especially where the second well layer is disposed between the first well layer and the p-type semiconductor layer, light emitted by the first well layer is absorbed in the second well layer so that its luminous intensity of the first well layer is reduced. Therefore the light emitting device with desired color rendering property can be easily obtained by depositing the first well layers more times than the second well layer. Also where the growth number of the second well layers that has relatively worse crystallinity is fewer, advantageously the p-type semiconductor layer can be formed subsequently on the second well layer.
The light emitting device according to another aspect of the present invention, is characterized in that the active layer further including, a first barrier layer made of nitride compound semiconductor containing Al, the second barrier layer grown on each of the first and second well layers, a second barrier layer made of nitride compound semiconductor substantially not containing Al, the second barrier layer grown on said first barrier layer.
The active layer further comprises the first and second barrier layer so that the light emitting device has the forward voltage which is dramatically reduced in comparison with the conventional one. That is to say, the Vf is reduced from 3.5V to 3.0V according to the present invention. In conjunction with the reduced Vf, the light emitting device is also improved in the luminous intensity and the luminous efficiency.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the sprit and scope of the invention will become apparent to those skilled in the art from this detailed description.